Apparatus and method for distilling waste liquids

ABSTRACT

A distilling apparatus and method use a two step distillation and purification process for processing a waste liquid, such as an impure sulfuric acid solution, to form a highly concentrated sulfuric acid solution. First, the waste liquid is stored in a concentrating column, where it is heated. A condenser, which uses the waste liquid as a cooling medium, condenses the vapor generated by the heater. The condensed vapor is passed through a filter, which separates impurities out of the waste liquid, prior to feeding the waste liquid back into the concentrating column. Water is then removed from the waste liquid via a distilling process. The resulting concentrated liquid is then fed to a purifying column, where it is again heated, to remove residue, and condensed, resulting in a highly pure waste liquid. The liquid level and physical state of the waste liquid in both the concentrating column and the purifying column are monitored to insure that only an optimum amount of waste liquid for distillation is stored in each of the columns.

This application is a division of Ser. No. 08/992,086, filed Dec. 17,1997, now U.S. Pat. No. 6,508,915.

BACKGROUND OF THE INVENTION

The present invention relates to a distilling apparatus for distillingand purifying waste liquids such as sulfuric acid employed for chemicaltreatment in processes for producing semiconductors, etc.

As semiconductor devices such as LCD (Liquid Crystal Display) areproduced in increasing numbers, huge amounts of solutions, such as ofsulfuric acid, are employed. To achieve distillatory purification ofwaste liquids employed in production processes efficiently andaccurately reduces production costs and contributes to environmentalconservation.

In a process of producing semiconductor devices, for example, in a stepof cleaning wafers, a solution prepared by adding about 2% by weight (wt%) of hydrogen peroxide to about 97 wt % of a commercially availablesulfuric acid and heated to 80 to 120° C. is employed. When the wasteliquid left after use of the solution is to be recovered, the addedhydrogen peroxide is partly converted into water, and also, watermigrates into the solution. Accordingly, the concentration of thesulfuric acid in the waste liquid drops to 80 to 90 wt %. Further,impurities such as silicon, iron, sodium and organic materials arecontained in the waste liquid. The recovered waste liquid is sold orrecycled by subjecting it to distillatory purification under atmosphericpressure. FIG. 1 shows an example of a prior art sulfuric aciddistilling apparatus for distilling and purifying a sulfuric acid wasteliquid.

As shown in FIG. 1, the sulfuric acid distilling apparatus is equippedwith a concentrating column 101 and a purifying column 102. A contacttype liquid level sensor 103 is disposed in the concentrating column101. A valve 104 is opened and closed based on detection results of theliquid level sensor 103 such that a fixed amount of sulfuric acid wasteliquid may be in the concentrating column 101. The waste liquid suppliedto the concentrating column 101 is heated by a heater 105 to apredetermined temperature required to achieve distillation (e.g., 300°C. under the atmospheric pressure). The water content formed bydistillation is condensed by a condenser 106, stored in a tank 107, anddischarged by opening a valve 108. Water is supplied as a cooling mediumto the condenser 106. The sulfuric acid thus concentrated is supplied tothe purifying column 102.

The concentrated sulfuric acid is heated to a predetermined temperature(about 300° C. under atmospheric pressure) by a second heater 109provided in the purifying column 102, and thus a purified sulfuric acidsolution is formed. The purified sulfuric acid solution is passedthrough a condenser 110, stored in a tank 111 and can be discharged byopening a valve 112. Water is supplied as a cooling medium to thecondenser 110. The residue in the purifying column 102 left, afterpurification, is condensed by a condenser 113 and stored in a tank 114.The residue in the tank 114 is discharged by opening a valve 115.

It is important to control the liquid level such that the amount ofsulfuric acid waste liquid in the concentrating column 101 is constantin order to carry out correct control of distillation in theconcentrating column 101. However, in the concentrating column 101, whenthe solution is heated, the hydrogen peroxide contained in the solutionis vaporized to form foams, and also a large amount of hydrogen peroxideis decomposed to form oxygen foams. Such foaming greatly changes thelevel of the sulfuric acid waste liquid. Accordingly, the liquid levelsensor 103 may erroneously detect the liquid level, making it is verydifficult to control the amount of solution in the concentrating column101.

Further, since the sulfuric acid in the concentrating column 101 isheated to about 300° C. or higher, related parts in the concentratingcolumn 101 should have sufficient resistance to high temperatures of300° C. or higher. Further, when the concentrating column 101 is heatedto a high temperature, leakage at junctions and damage of the junctionscan be induced by heat shock.

Japanese Unexamined Patent Publication No. 61-291407 discloses a processin which hydrogen peroxide contained in a sulfuric acid waste liquid isremoved before the waste liquid is treated. According to this process,hydrogen peroxide is preliminarily removed from the sulfuric acid wasteliquid by adding a sulfurous acid gas in an amount equal to or more thanthe equivalent amount of hydrogen peroxide to effect a chemicalreaction. This process can prevent oxidation of a membrane for filteringthe sulfuric acid waste liquid by hydrogen peroxide. However, thesulfurous acid does not entirely react with the hydrogen peroxide, andcan form a reaction product of sulfuric acid and sulfurous acid. Thereaction product is present in the sulfuric acid waste liquid as animpurity. As a result, the state of the sulfuric acid waste liquid ischanged to lower the sulfuric acid purification accuracy.

The present invention provides a distilling apparatus which facilitatescontrol of the amount of waste liquid.

SUMMARY OF THE INVENTION

Briefly stated, the present invention provides a distilling apparatusincluding: a distiller which stores therein a solution to be treated anddistills the solution to form a distillate; a liquid level detectordisposed in the distiller which detects the liquid level of the to betreated solution; a state detector for detecting a physical state of theto be treated solution contained in the distiller; and a control unitfor controlling the amount of to be treated solution stored in thedistiller based on detection results of the liquid level detector andthe state detector.

The present invention further provides a distilling apparatus fordistilling a hydrogen peroxide-containing solution to be treated. Theapparatus includes: a remover for removing a substantial part of thehydrogen peroxide from the to be treated solution; a distiller,connected to the remover, for storing therein the to be treated solutionfrom which the hydrogen peroxide has been removed and distilling thesolution to form a distillate; and a control unit for controlling theamount of to be treated solution stored in the distiller based on aliquid level of the to be treated solution in the distiller.

The present invention provides a method for distilling a solution to betreated. The method includes the steps of: storing the to be treatedsolution in a distiller; detecting a liquid level of the to be treatedsolution stored in the distiller with a liquid level detector disposedin the distiller; detecting a physical state of the to be treatedsolution in the distiller with a state detector; controlling the amountof to be treated solution to be supplied to the distiller based on theliquid level and physical state detection results; and heating the to betreated solution stored in the distiller to form a distillate.

The present invention further provides a method for distilling ahydrogen peroxide-containing solution to be treated. The method includesthe steps of: removing a substantial part of the hydrogen peroxide fromthe to be treated solution; storing the to be treated solution fromwhich hydrogen peroxide has been removed in a distiller; controlling theamount of to be treated solution supplied to the distiller based on aliquid level of the to be treated solution in the distiller; and heatingthe to be treated solution stored in the distiller to form a distillate.

Other aspects and advantages of the invention will become apparent fromthe following description, taken in conjunction with the accompanyingdrawings, illustrating by way of example the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best beunderstood by reference to the following description of the presentlypreferred embodiments together with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a prior art sulfuric acid distillingapparatus;

FIG. 2 is a schematic diagram of a distilling apparatus according to afirst embodiment of the present invention;

FIG. 3 is also a schematic diagram of the distilling apparatus accordingto the first embodiment of the present invention;

FIG. 4A is a schematic cross-sectional side view of a concentratingcolumn of the apparatus of FIG. 2, and FIG. 4B is a horizontalcross-sectional view of the concentrating column of the apparatus ofFIG. 2;

FIGS. 5A and 5B are schematic diagrams showing actions of the purifyingcolumn of FIG. 3;

FIG. 6 is a schematic diagram of a distilling apparatus according to asecond embodiment of the present invention;

FIGS. 7A and 7B are partial schematic diagram of third distillingapparatus;

FIG. 8 is a partial schematic diagram of fourth distilling apparatus;

FIG. 9 is a partial schematic diagram of fifth distilling apparatus; and

FIG. 10 is a partial schematic diagram of sixth distilling apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Certain terminology is used herein for convenience and is not meant tobe construed narrowly or limiting. The word “substantial” generallymeans greater than half. The terminology includes the word abovespecifically mentioned, derivatives thereof and words similar import. Inthe drawings, like numerals indicate like elements throughout.

(First Embodiment)

A distillatory purifying apparatus according to a first embodiment ofthe present invention will be described referring to the drawings. FIGS.2 and 3 show schematic diagrams of the distillatory purifying apparatus.The distillatory purifying apparatus of the first embodiment, whichincludes a concentrating column 1 shown in FIG. 2 and a purifying column2 shown in FIG. 3, carries out a two step distillatory purificationprocess of a waste liquid, such as a sulfuric acid waste liquid.

As shown in FIG. 2, the concentrating column 1 has a column head 1 acontaining a condenser 3. A waste liquid, referred to herein as asulfuric acid waste liquid, is supplied as a cooling medium (coolant)from a waste liquid tank (not shown) to the condenser 3. The sulfuricacid waste liquid is, for example, one employed in a step of cleaningsemiconductor wafers, such as a sulfuric acid solution containing about2% by weight (wt %) of aqueous hydrogen peroxide as an oxidizing agent,or a sulfuric acid waste liquid employed in an LCD production processmay be supplied to the concentrating column 1.

The sulfuric acid waste liquid serves as a cooling medium for condensingthe vapor in the concentrating column 1. Accordingly, the vapor in theconcentrating column 1 is cooled by heat exchange with the sulfuric acidwaste liquid, so that the waste liquid is heated to a predeterminedtemperature (e.g., 30 to 80° C.). When the sulfuric acid waste liquid isheated, the hydrogen peroxide contained therein is partly vaporized andpartly decomposed into water and oxygen. If the condenser 3 is damaged,the cooling medium can migrate into the concentrating column 1. However,since the cooling medium according to the present invention is thesulfuric acid waste liquid, the sulfuric acid waste liquid in theconcentrating column 1 does not react with the vapor.

The sulfuric acid waste liquid passed from the condenser 3 is suppliedthrough a filter 4 and a supply valve 5 into the concentrating column 1.The filter 4 is a filter having a hydrophobic membrane, preferably aTeflon filter. The filter 4 separates the vaporized hydrogen peroxideand decomposition product oxygen from the sulfuric acid waste liquid(gas-liquid separation). Accordingly, the vaporized hydrogen peroxideand the decomposition product oxygen preliminarily removed from thesulfuric acid waste liquid supplied to the concentrating column 1. Thatis, in the first embodiment, no extra substance is added to removehydrogen peroxide. Thus, no reaction product is formed, and the state ofthe sulfuric acid waste liquid remains unchanged, which maintainspurification accuracy at a high level.

The hydrogen peroxide separated from the sulfuric acid waste liquid maybe reacted with sulfur dioxide to form sulfuric acid, and the thusformed sulfuric acid may be added to the sulfuric acid waste liquidbefore it is subjected to distillation. In this case, the concentrationof sulfuric acid in such untreated sulfuric acid waste liquid isincreased to form a purified sulfuric acid with higher purity.

A water seal vacuum pump 6 is preferably connected to the concentratingcolumn 1. However, other vacuum pumps, such as an oil-sealed rotaryvacuum pump or a reciprocating pump may be used.

The vacuum pump 6 reduces the internal pressure of the concentratingcolumn 1 to a predetermined level. In the first embodiment, the vacuumpump 6 is designed to reduce the internal pressure of the concentratingcolumn 1 preferably to 50 Torr or less, and more preferably to about 5to 20 Torr. A buffer 7 is interposed between the concentrating column 1and the vacuum pump 6.

A heater 8 for heating the sulfuric acid waste liquid is disposed in theconcentrating column 1. The heater 8, the external surface of which ispreferably protected with quartz, is directly inserted laterally intothe column 1, so as to increase the efficiency of heating the sulfuricacid waste liquid. The sulfuric acid waste liquid supplied to theconcentrating column 1 contains no hydrogen peroxide, since it has beenremoved beforehand. Accordingly, the sulfuric acid waste liquid heatedby the heater 8 is controlled so as not to undergo foaming. In otherwords, the sulfuric acid waste liquid is heated and distilled relativelymildly in the concentrating column 1.

Further, since the internal pressure of the concentrating column 1 isreduced by the vacuum pump 6, the boiling point of the sulfuric acidwaste liquid is lower than under atmospheric pressure. For example, whenthe internal pressure of the concentrating column 1 is reduced to about5 to 20 Torr, the boiling point of the sulfuric acid waste liquid isabout 180 to 220° C. Accordingly, the sulfuric acid waste liquid isheated to a temperature (about 180 to 220° C.) which is lower than thetemperature (300° C. or higher) to which it is heated under atmosphericpressure, which reduces the load of the heater 8. The sulfuric acidwaste liquid is heated to 30 to 80° C. by heat exchange when it passesthe condenser 106 and then supplied to the concentrating column 1.Accordingly, the heater 8 heats the preheated sulfuric acid waste liquidto 180 to 220° C., which further reduces the load of the heater 8.Further, there is no to employ parts having high-temperature resistancein the concentrating column 1. In addition, since rise in thetemperature of the parts employed is controlled, the liability ofleakage at junctions or damage of the junctions induced by heat shock isreduced.

The concentrating column 1 has a liquid level sensor 9, a temperaturesensor 10 a, a concentration sensor 10 b and a specific gravity sensor10 c. The sensors 9 and 10 a to 10 c are connected to a control unit 12.The liquid level sensor 9 is preferably a continuous capacitance sensorwhich detects the level of the sulfuric acid waste liquid in the column1. The temperature sensor 10 a, concentration sensor 10 b and specificgravity sensor 10 c detect the temperature, concentration and specificgravity of the sulfuric acid waste liquid in the column 1, respectively.

The control unit 12 receives signals expressing detection results fromthe sensors 9 and 10 a to 10 c respectively, and calculates an optimumvolume of sulfuric acid waste liquid for carrying out distillation,based on the detection results of the sensors, and finds a targetcapacitance value of the liquid level sensor 9, depending on thecalculated optimum volume. The control unit 12 further controls openingand closing of the supply valve 5 so that the actual capacitance valueof the liquid level sensor 9 substantially coincides with the targetcapacitance value when the sulfuric acid waste liquid is supplied intothe concentrating column 1. The sulfuric acid waste liquid contains nohydrogen peroxide, as previously discussed, since it has been removedbefore the waste liquid is supplied to the concentrating column 1, sothat the waste liquid undergoes less foaming during heating, and thusthere is less induced vibration on the liquid surface. Therefore, thelevel of the sulfuric acid waste liquid can be detected accurately bythe liquid level sensor 9. Thus, the level of the sulfuric acid wasteliquid can be controlled accurately, depending on the temperature andpressure.

The concentrating column 1 contains partitions 13 for separating aninner space thereof. As shown in FIG. 4A, each partition 13 has side endfaces conforming to the profile of the inner wall surface of theconcentrating column 1 and an upper end face formed such that it issubstantially at a maximum level of the sulfuric acid waste liquidsupplied into the concentrating column 1. In the first embodiment, fourpartitions 13 are connected by two connecting parts 14, such that thepartitions 13 are retained at equal intervals, as shown in FIG. 4B. Thefour partitions 13 divide the inner solution storing space of theconcentrating column 1 into a plurality of spaces. Each partition 13makes the sulfuric acid waste liquid present in one of the thus dividedspaces difficult to flow into the other divided spaces. That is,vibration at the liquid surface in one divided space is not readilytransmitted to the liquid surfaces in the other divided spaces. In otherwords, the partitions 13 serve as restricting plates for restrictingmigration of the sulfuric acid waste liquid, and thus reduce vibrationat the liquid surface.

As shown in FIG. 4A, each partition 13 contains a plurality of pores 15.The pores 15 are preferably formed at a lower part of each partition 13.The pores 15 absorb foams formed when the sulfuric acid waste liquid isboiled. The pores 15 are effective for suppressing rippling of thesulfuric acid waste liquid and thus allow accurate detection of theliquid level.

As shown in FIG. 2, a cylindrical filler (rasching ring) 16 for coolingthe vapor formed during distillation is disposed in the concentratingcolumn 1. The vapor contains sulfuric acid vapor and water vapor. Sincethe boiling point of sulfuric acid vapor is higher than that of watervapor, the sulfuric acid vapor is liquefied to drop when it is cooled bythe rasching ring 16. The water vapor passes the rasching ring 16 and isintroduced to the condenser 3. The water vapor is condensed by thecondenser 3, and the liquefied water content is stored in a receivingcontainer 17. Thus, the concentration of sulfuric acid contained in thesulfuric acid waste liquid in the concentrating column 1 is increased.

The receiving container 17 contains a primary container 17 a and asecondary container 17 b. A primary discharge valve 18 is interposedbetween the containers 17 a and 17 b, and a secondary discharge valve 19is disposed below the secondary container 17 b. The secondary container17 b is connected via a vacuum valve 20 to the buffer 7. When the vacuumvalve 20 is opened, the internal pressure of the secondary container 17b is reduced to the same predetermined level as the pressure in theconcentrating column 1. The secondary container 17 b contains anatmospheric pressure valve 21 for opening the inner space of thecontainer 17 b to the atmosphere.

The water content condensed by heat exchange by the condenser 3 is firststored in the primary container 17 a of the receiving container 17 andthen transferred to the secondary container 17 b under opening andclosing operation of the primary discharge valve 18. In this step, thevacuum valve 20 is opened, which reduces the internal pressure of thesecondary container 17 b. Accordingly, when the water content stored inthe primary container 17 a is transferred to the secondary container 17b, the internal pressure of the concentrating column 1 is maintained ata fixed level. Further, the buffer 7 absorbs pressure fluctuationoccurring when the water content is transferred from the primarycontainer 17 a to the secondary container 17 b. Accordingly, theinternal pressure of the concentrating column 1 is maintained at a fixedlevel.

The water content stored in the secondary container 17 b is dischargedin the following order. First, the primary valve 18 and the vacuum valve20 are closed. Next, the atmospheric pressure valve 21 is opened.Finally, the secondary discharge valve 19 is opened. Since the primaryvalve 18 is closed when the water content is discharged, the internalpressure of the concentrating column 1 is maintained at a fixed level.Further, since the inner space of the secondary container 17 b is opento the atmosphere, the water content stored therein is speedilydischarged.

As shown in FIGS. 2 and 3, the concentrating column 1 and the purifyingcolumn 2 are connected to each other via a connecting pipe 22 having asupply valve 23. When the sulfuric acid waste liquid is subjected todistillation in the concentrating column 1, the water content isevaporated to form a sulfuric acid solution (concentrated sulfuric acid)having a predetermined concentration (about 97 wt %). The sulfuric acidsolution is transferred from the concentrating column 1 through theconnecting pipe 22 to the purifying column 2 by opening the supply valve23, due to the difference between the specific gravity of the liquid inthe concentrating column 1 and that of the liquid in the purifyingcolumn 2.

The purifying column 2 is connected to the vacuum pump 6 via the buffer7 and is maintained at a predetermined reduced pressure by the vacuumpump 6 and the buffer 7. The purifying column 2 has a heater 24 forheating the concentrated sulfuric acid. The heater 24, which ispreferably protected on the external surface with quartz, is directlyinserted laterally into the column 2.

Since the internal pressure of the purifying column 2 is reduced, theboiling point of the concentrated sulfuric acid is lower than theboiling point under atmospheric pressure. For example, when the internalpressure of the purifying column 2 is reduced to about 5 to 20 Torr, theboiling point of the sulfuric acid is lowered to about 180 to 220° C.Accordingly, the heater 24 heats the sulfuric acid to a temperature(about 180 to 220° C.) which is lower than the temperature (300° C. orhigher) required under atmospheric pressure. This reduces the load ofthe heater 24.

The purifying column 2 has a liquid level sensor 25, a temperaturesensor 26 a, a concentration sensor 26 b and a specific gravity sensor26 c, which are all connected to the control unit 12. The liquid levelsensor 25 is preferably a continuous capacitance sensor which detectsthe level of the sulfuric acid in the column 2 and outputs a signalexpressing the liquid level of the sulfuric acid solution. Thetemperature sensor 26 a, concentration sensor 26 b and specific gravitysensor 26 c detect the temperature, concentration and specific gravityof the sulfuric acid waste liquid in the column 2 and output signalsexpressing related detection results, respectively.

The control unit 12 receives the signals from the sensors 25 and 26 a to26 c and calculates an optimum volume of the sulfuric acid for carryingout purification and determines the target capacitance value of theliquid level sensor 25, depending on the calculated optimum volume. Thecontrol unit 12 controls opening and closing of the supply valve 23 sothat the actual capacitance value of the liquid level sensor 25substantially coincides with the target capacitance value, when thesulfuric acid is supplied into the concentrating column 2. Thus, thelevel of the sulfuric acid is controlled accurately depending on thetemperature and pressure.

A rasching ring 28, as a filter for cooling the vapor (sulfuric acidgas), is disposed in the purifying column 2. The sulfuric acid vaporcontains impurities, which deteriorate the quality of the purifiedsulfuric acid. The rasching ring 28 removes impurities from the sulfuricacid vapor by liquefying the sulfuric acid vapor partly, to drops. Thegreatest part of the vapor passes the rasching ring 28 and is introducedfrom the column head 2 a to a condenser 29.

The sulfuric acid waste liquid which is not yet distilled is supplied asa cooling medium to the condenser 29. The sulfuric acid vapor undergoesheat exchange with the sulfuric acid waste liquid when the former passesthe condenser 29. Thus, the sulfuric acid waste liquid is heated, whilethe sulfuric acid vapor is condensed to assume a liquid form. Thesulfuric acid liquid is transferred to a second condenser 30 andcondensed there. The untreated sulfuric acid waste liquid is alsosupplied as a cooling medium to the second condenser 30. The sulfuricacid liquid condensed by the second condenser 30 the waste liquid storedas a purified sulfuric acid in a receiving container 31.

Since the untreated sulfuric acid waste liquid is employed as a coolingmedium for the condensers 29 and 30, it does not react with the purifiedsulfuric acid even if the condensers 29 and 30 are damaged and the wasteliquid migrates into the purified acid in the receiving container 31.

The receiving container 31 includes a primary container 31 a and asecondary container 31 b. A primary discharge valve 32 is interposedbetween the containers 31 a and 31 b, and a secondary discharge valve 33is connected to the secondary container 31 b. The secondary container 31b is connected to the buffer 7. A vacuum valve 34 is interposed betweenthe secondary container 31 b and the buffer 7. When the vacuum valve 34is opened, the internal pressure of the secondary container 31 b isreduced to the same predetermined level as the pressure in the purifyingcolumn 2. The secondary container 31 b contains an atmospheric pressurevalve 35 for opening an inner space of the container 31 b to theatmosphere.

The purified sulfuric acid condensed by the condensers 29 and 30 isfirst stored in the primary container 31 a of the receiving container 31and then transferred to the secondary container 31 b by opening theprimary discharge valve 32. When the purified sulfuric acid istransferred, the vacuum valve 34 is opened, so that the internalpressure of the secondary container 31 b is reduced. Accordingly, theinternal pressure of the purifying column 2 is maintained at a fixedlevel, when the purified sulfuric acid stored in the primary container31 a is transferred to the secondary container 31 b. Further, the buffer7 absorbs pressure fluctuation occurring when the purified sulfuric acidis transferred from the primary container 31 a to the secondarycontainer 31 b. Thus, the internal pressure of the purifying column 2 ismaintained at a fixed level.

The purified sulfuric acid stored in the secondary container 31 b isdischarged in the following order. First, the primary valve 32 and thevacuum valve 34 are closed. Next, the atmospheric pressure valve 35 isopened to open the inner space of the secondary container 31 b to theatmosphere. Finally, the secondary discharge valve 33 is opened todischarge the purified sulfuric acid. Since the primary discharge valve32 is closed when the purified sulfuric acid is discharged, the internalpressure of the purifying column 2 is maintained at a fixed level.Further, since the inner space of the secondary container 31 b is opento the atmosphere, the purified sulfuric acid stored therein isdischarged speedily.

The purifying column 2 is provided with a metering discharge mechanism36 which adjusts the amount of residue-containing liquid (concentratedsolution) discharged from the purifying column 2 so as to maintain highpurity of the sulfuric acid in the purifying column 2. The residuecontains impurities. The metering discharge mechanism 36 increases thedischarge amount of concentrated sulfuric acid when the specific gravityof the liquid in the purifying column 2 is high and decreases thedischarge amount when the specific gravity thereof is low. Thus, theresidue is discharged at a predetermined ratio relative to the purifiedsulfuric acid discharged from the secondary container 31 b. In the firstembodiment, the residue is discharged in an amount of about 10% relativeto the purified sulfuric acid.

The metering discharge mechanism 36 includes a resilient body 37 forurging the purifying column 2 upward, a discharge pipe 38 connected tothe purifying column 2 and a discharge pipe 39 connected to thedischarge pipe 38. A condenser 40 for condensing the residue isinterposed between the discharge pipe 38 and the discharge pipe 39.

The resilient body 37 preferably includes springs disposed under thepurifying column 2 which support the column 2. Under the action of thesprings, the purifying column 2 sinks when the concentration of theresidue is increased (when the specific gravity of the liquid in thepurifying column 2 is high), while it rises when the concentration ofthe residue is reduced (when the specific gravity thereof is low). Thatis, the purifying column 2 floats up and down depending on the weight ofthe purifying column 2 and the weight of the liquid (sulfuric acid andresidue) present therein. The discharge pipe 38 also preferably floatsup and down together with the purifying column 2, and the liquiddischarged from the purifying column 2 flows through the pipe 38 towardthe discharge pipe 39.

The discharge pipe 39 is connected by a control mechanism 200 whichcauses the discharge pipe 39 to move up and down in accordance with theup and down floating of the purifying column 2. Accordingly, the controlmechanism 200 causes the discharge pipe 39 to move down when thepurifying column 2 moves down and to move up when the purifying column 2moves up, so that the portion of the liquid which is higher than thedischarge pipe 39 is discharged through the discharge pipes 38, 39. Thatis, the control mechanism 200 controls the position of the dischargepipe 39 in accordance with the up and down floating of the entirepurifying column 2. The residue is discharged at a predetermined ratio.The control mechanism 200 is implemented by applying, for example, aprinciple of lever. For example, when the concentration of the residue(impurities) in the liquid sulfuric acid is high, as shown in FIG. 5A,the purifying column 2 sinks (downward in FIG. 5A) against the urgingforce of the resilient body 37. Then, the control mechanism 200 movesdown the discharge pipe 39 with the downward movement of the purifyingcolumn 2. Thus, the liquid level becomes higher than the fixed position(as indicated by the dash-dotted line in FIG. 5A) of the discharge pipe39. Consequently, a large amount of liquid is discharged through thedischarge pipe 39.

Meanwhile, when the concentration of the residue in the sulfuric acid islow, as shown in FIG. 5B, the purifying column 2 moves up (upward inFIG. 5B) by the urging force of the resilient body 37. Then, the controlmechanism 200 moves up the discharge pipe 39 with the upward movement ofthe purifying column 2. Thus, the liquid level becomes slightly higherthan the fixed position (as indicated by the dash-dotted line in FIG.5B) of the discharge pipe 39. Consequently, a small amount of liquid isdischarged through the discharge pipe 39. As shown in FIGS. 5A and 5B,the discharge pipe 38 and the discharge pipe 39 are connected by aflexible pipe 41. In FIGS. 5A and 5B, the condenser 40, shown in FIG. 3,is omitted.

As shown in FIG. 3, the residue-containing concentrated sulfuric aciddischarged from the purifying column 2 is stored in a receivingcontainer 42. The receiving container 42 includes a primary container 42a and a secondary container 42 b. A primary discharge valve 43 isinterposed between the containers 42 a and 42 b and a secondarydischarge valve 44 is connected to the secondary container 42 b. Thesecondary container 42 b is connected to the buffer 7. A vacuum valve 45is interposed between the secondary container 42 b and the buffer 7.When the vacuum valve 45 is opened, the internal pressure of thesecondary container 42 b is reduced to the same predetermined level asin the purifying column 2. The secondary container 42 b contains anatmospheric pressure valve 46 for opening the inner space of thecontainer 42 b to the atmosphere.

The residue-containing concentrated sulfuric acid condensed by thecondenser 40 is first stored in the primary container 42 a and thentransferred to the secondary container 42 b by opening the primarydischarge valve 43. When the residue-containing sulfuric acid istransferred, the vacuum valve 45 is opened, so that the internalpressure of the purifying column 2 is maintained at a fixed level.Further, the buffer 7 of the vacuum system absorbs pressure fluctuationoccurring when the residue-containing sulfuric acid is transferred fromthe primary container 42 a to the secondary container 42 b.

The residue-containing sulfuric acid is discharged from the secondarycontainer 42 b in the following order. First, the primary valve 43 andthe vacuum valve 45 are closed. Next, the atmospheric pressure valve 46is opened, to open the inner space of the secondary container 42 b tothe atmosphere. Finally, the secondary discharge valve 44 is opened todischarge the residue-containing sulfuric acid. Since the primarydischarge valve 43 is closed when the residue-containing sulfuric acidis discharged, the internal pressure of the purifying column 2 ismaintained at a fixed level. Further, since the inner space of thesecondary container 42 b is open to the atmosphere, theresidue-containing sulfuric acid is discharged speedily.

In the first embodiment, the hydrogen peroxide separated beforehand fromthe sulfuric acid waste liquid may be bubbled through the liquidcontaining water, residues, etc., stored in the secondary container 17b, 31 b or 42 b. The liquid stored in the secondary container 17 b, 31 bor 42 b contains a large amount of sulfur dioxide which reacts withhydrogen peroxide to form sulfuric acid. As a result, the hydrogenperoxide separated can be utilized effectively to control discharge ofharmful sulfur dioxide.

(Second Embodiment)

Next, a distillatory purifying apparatus according to a secondembodiment of the invention will be described. FIG. 6 shows a schematicdiagram of a distillatory purifying apparatus for distilling andpurifying a waste liquid, such as nitric acid. The distillatorypurifying apparatus contains a purifying column 51, a first cooler 52, asecond cooler 53, an exhaust chamber 54, a denitrating column 55, aproduct cooler 56, a waste acid cooler 57, a receiving container 58 anda control unit 59. The control unit 59 performs various controls in thedistilling treatment of the nitric acid waste liquid.

In the second embodiment, the nitric acid waste liquid is distilled andpurified in the purifying column 51 to form a purified nitric acid. Morespecifically, the purifying column 51 has both the function of theconcentrating column 1 shown in FIG. 2 and the function of the purifyingcolumn 2 shown in FIG. 3. A purified nitric acid having the desiredconcentration cannot sometimes be formed by the treatment in thepurifying column 51 only, depending on the concentration of the nitricacid waste liquid. In this case, the concentrating column 1 shown inFIG. 2 is employed to concentrate the nitric acid waste liquid to supplythe resulting concentrated nitric acid to the purifying column 51. Thus,a purified nitric acid having the desired concentration can be obtained.Since the nitric acid waste liquid has a low boiling point underatmospheric pressure, the requirement of reducing the internal pressureof the purifying column 51 is not necessary.

The nitric acid waste liquid is supplied as a cooling medium (coolant)from a waste liquid tank (not shown) to the first cooler 52. The nitricacid waste liquid is subjected to heat exchange with the vapor in thefirst cooler 52. Thus, the vapor in the first cooler 52 is condensed,and the nitric acid waste liquid is heated to a predeterminedtemperature (e.g., about 30 to about 80° C.).

The thus heated nitric acid waste liquid is supplied through a supplyvalve 61 to the purifying column 51. A heater 62 for heating andpreferably boiling the nitric acid waste liquid is disposed in thepurifying column 51. The heater 62, the external surface of which ispreferably protected, such as with quartz, is directly insertedlaterally into the column 51, so as to enhance the efficiency of heatingthe nitric acid waste liquid. The heater 62 may heat the nitric acidwaste liquid, preheated to 30 to 80° C. by heat exchange, to about 100to about 120° C. The preheating via heat exchange reduces the load ofthe heater 62.

The purifying column 51 has a liquid level sensor 63, a temperaturesensor 64 a, a concentration sensor 64 b and a specific gravity sensor64 c, which are connected to the control unit 59. The liquid levelsensor 63 is preferably a continuous capacitance sensor which outputs asignal expressing the level of the nitric acid waste liquid in thepurifying column 51. The temperature sensor 64 a, concentration sensor64 b and specific gravity sensor 64 c output signals expressing thetemperature, concentration and specific gravity of the nitric acid wasteliquid in the purifying column 51, respectively.

The control unit 59 receives the sensor signals from the sensors 63 and64 a to 64 c to recognize the state of the nitric acid waste liquid inthe purifying column 51. The control unit 59 calculates an optimumvolume of the nitric acid waste liquid for carrying out concentrationand purification based on the detection results of the sensors anddetermines the target value of the level of the nitric acid waste liquidin the purifying column 51, based on the calculated volume optimum. Thecontrol unit 59 controls opening and closing of the supply valve 61 sothat the liquid level in the purifying column 51 coincides with thedetermined target value, when the nitric acid waste liquid is suppliedinto the concentrating column 51. The liquid level in the purifyingcolumn 51 can be controlled accurately by the control unit 59, dependingon the conditions, such as temperature.

The purifying column 51 is connected to the control unit 59 and includesa pressure sensor 65 which outputs a signal expressing the internalpressure of the purifying column 51. The control unit 59 receives thepressure sensor signal to monitor the internal pressure of the purifyingcolumn 51. The control unit 59, upon recognition of an abrupt rise inthe internal pressure of the purifying column 51, judges that someabnormality has occurred in the distillatory purifying apparatus andbehaves to stop operation of the apparatus.

The purifying column 51 has a second temperature sensor 66 disposed inthe column head 51 a thereof. The second temperature sensor 66 isconnected to the control unit 59 and outputs a signal expressing theinternal temperature of the column head 51 a. The control unit 59receives the temperature signal from the temperature sensor 66 tomonitor the temperature at the column head 51 a. The control unit 59adjusts the flow rate of the nitric acid waste liquid supplied to thefirst cooler 52 such that the temperature at the column head 51 a isgenerally constant.

The purifying column 51 contains a plurality of partitions 13 which areessentially the same as the partitions in the first embodiment. Sincethe partitions 13 have already been described in the first embodiment, adetailed description herein is omitted. As before, each partition 13contains a plurality of pores 15 formed therethrough (see FIG. 4A).

The vapor (nitric acid gas) in the purifying column 51 is cooled by afiller 67 disposed therein and then introduced from the column head 51 ato the first cooler 52, where the nitric acid gas undergoes heatexchange with the untreated nitric acid waste liquid serving as acooling medium. Thus, the nitric acid waste liquid is heated, and thenitric acid gas is condensed to assume a liquid form, i.e., into apurified nitric acid. The purified nitric acid is introduced through apipe 68 to the denitrating column 55. The pipe 68 is provided with acheck valve 69 for preventing back flow of the purified nitric acid fromthe denitrating column 55 to the first cooler 52.

The purified nitric acid in the denitrating column 55 reacts with airsupplied thereto and is converted from a brown liquid to a colorlesstransparent liquid. The transparent purified nitric acid thus formed issupplied to the product cooler 56, where it is cooled by cold water andstored in a product container (not shown).

The pipe 68 is provided with a temperature sensor 70 connected to thecontrol unit 59 which outputs a signal expressing the temperature of thepurified nitric acid flowing through the pipe 68. The control unit 59receives the signal from the temperature sensor 70 and monitors thetemperature of the purified nitric acid in the denitrating column 55.The control unit 59 adjusts the amount of air supplied to thedenitrating column 55 depending on the temperature of the purifiednitric acid.

A gas containing NO_(x) (nitric acid mist) is formed in the first cooler52. The gas is cooled by cold water supplied to the second cooler 53 andthen the gas is introduced to the exhaust chamber 54 to which the aircontaining NO_(x) (nitric acid mist) formed by the reaction between airand nitric acid in the denitrating column 55 is introduced.

The exhaust chamber 54 is packed with a catalyst 71 for removing NO_(x).The catalyst 71 adsorbs NO_(x) contained in the gas introduced to theexhaust chamber 54 so that a purified gas is exhausted from the exhaustchamber 54.

The purifying column 51 is provided with the same metering dischargemechanism 36 as in the first embodiment. The metering dischargemechanism 36 adjusts the amount of residue-containing liquid such thatresidue may be discharged from the purifying column 51 at apredetermined ratio relative to the purified nitric acid. Since themetering discharge mechanism 36 has already been described in the firstembodiment, further description is omitted. The discharge pipe 38 isprovided with the waste acid cooler 57 for cooling the nitric acid wasteliquid to be discharged.

A temperature sensor 72 is provided in the route of discharging thenitric acid waste liquid, preferably on the discharge pipe 39. Thetemperature sensor 72 is connected to the control unit 59 and outputs asignal expressing the temperature of the nitric acid waste liquid. Thecontrol unit 59 receives the signal from the temperature sensor 72 andmonitors the temperature of the nitric acid waste liquid discharged fromthe purifying column 51. The control unit 59 recognizes an occurrence ofsome abnormality in the distillatory purifying apparatus when thetemperature of the waste acid exceeds a predetermined temperature andbehaves to stop operation of the apparatus.

The nitric acid waste liquid discharged is stored in the receivingcontainer 58. The receiving container 58 is connected to piping having adischarge valve 73. The receiving container 58 is provided with a liquidlevel sensor 74 which is connected to the control unit 59 and outputs asignal expressing the level of the nitric acid waste liquid stored inthe receiving container 58. More specifically, the liquid level sensor74 is a capacitive sensor and is fixed at a predetermined height. Theliquid level sensor 74 is designed such that its capacity may changedepending on the level of the nitric acid waste liquid. The control unit59 receives the signal from the liquid level sensor 74 and monitors theamount of nitric acid waste liquid stored in the receiving container 58.The control unit 59 opens the discharge valve 73 discharges the nitricacid waste liquid when the amount of nitric acid waste liquid reaches apredetermined level (e.g., the position where the liquid level sensor 74is attached). The control unit 59 counts the frequency that the nitricacid waste liquid has reached the predetermined level in response to thesignals from the liquid level sensor 74. The amount of nitric acid wasteliquid stored in the receiving container 58 is determined depending onthe position where the liquid level sensor 74 is attached. The controlunit 59 monitors the discharge amount of nitric acid waste liquid basedon the count.

In the second embodiment, the filter 4 as employed in the firstembodiment is interposed between the first cooler 52 and the supplyvalve 61. The filter 4 enables treatment of a nitric acid waste liquidcontaining hydrogen peroxide. The hydrogen peroxide contained in thenitric acid waste liquid is heated by heat exchange in the first cooler52 and vaporized. The filter 4 separates the vaporized hydrogen peroxidefrom the nitric acid waste liquid.

It should be apparent to those skilled in the art that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention. Particularly, it should beunderstood that the invention may be embodied in the following forms.

As shown in FIGS. 7A and 7B, a cylindrical container 91 containing acatalyst 92 (FIG. 7A) or 93 (FIG. 7B) may be interposed between asulfuric acid waste liquid tank and the filter 4. The catalyst 92 or 93decomposes hydrogen peroxide into oxygen and water, and the filter 4separates the oxygen from the hydrogen peroxide. As the catalyst, ametal having low reactivity with sulfuric acid, for example, gold orplatinum is preferably employed. The catalyst 92 or 93 preferably has agranular form, bar-like form, etc. FIG. 7A shows an example where bars,such as of gold or platinum, are employed as the catalyst 92. It ispreferred to use as the catalyst 92 platinum which is employed aselectrodes. FIG. 7B shows an example where a granular catalyst, such asgold or platinum, is employed as the catalyst 93. The catalyst 93 ishighly reactive with hydrogen peroxide to facilitate decomposition ofhydrogen peroxide, advantageously.

As shown in FIG. 8, a condenser 94 may be interposed between the wasteliquid tank and the filter 4 to heat the sulfuric acid waste liquid andremove hydrogen peroxide, prior to distillation treatment. As a heatingmedium supplied to the condenser 94, a warm water is preferably used.The sulfuric acid waste liquid may be heated, for example, utilizing theheat of reaction between the water content stored in the secondarycontainer 17 b or 31 b with the purified sulfuric acid. The watercontent is preferably supplied by the water seal vacuum pump 6. Suchsupply of water content reduces the amount of coolant employed.

As shown in FIG. 9, a condenser 95 may be provided on the pipe 22connecting the concentrating column 1 and the purifying column 2. When aheating medium is supplied to the condenser 95, the concentratedsulfuric acid flowing through the pipe 22 is heated. Since thetemperature of the sulfuric acid is increased by the condenser 95, theload of the heater in the concentrating column 1 can be reduced. Themixture and the purified sulfuric acid employed as the heating mediummay be returned to the tank. In this case, the concentration of sulfuricacid in the untreated sulfuric acid waste liquid is increased, so that apurified sulfuric acid having a high purity is obtained.

A piping may be inserted to the concentrating column 1, purifying column2 or purifying column 51 so that the untreated sulfuric acid wasteliquid (or untreated nitric acid waste liquid) may flow therethrough.The untreated sulfuric acid (or nitric acid) waste liquid is heated whenit flows through the piping, and hydrogen peroxide contained therein isremoved. As shown in FIG. 10, piping 96 may be wound around the heater8, 24 or 62. The piping 96 allows the sulfuric acid waste liquid flowingtherethrough to be efficiently heated by the heater 8.

The resilient body 37 may be disposed above the purifying column 2 or 51with the column being suspended therefrom. Further or additional,resilient bodies 37 may also be disposed above and under the purifyingcolumn 2 or 51, so that the purifying column 2 or 51 may be urgedupward.

The filter 4 having a hydrophobic membrane may be replaced with aseparator such as a chamber.

Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalence of the appended claims.

1. A distilling apparatus comprising: a distiller which stores therein asolution to be treated and distills the solution to form a distillate,wherein the distiller includes a cooler for cooling a distillation gasformed by heating the to be treated solution in the distiller, andwherein the distiller includes a plurality of partitions retained in thedistiller at equal intervals, each of the plurality of partitions havinga plurality of pores; a liquid level detector disposed in the distillerwhich detects the liquid level of the to be treated solution; a statedetector for detecting a physical state of the to be treated solutioncontained in the distiller, wherein state detector includes atemperature sensor, a concentration sensor, and a specific gravitysensor; and a control unit for setting a target liquid level of theliquid level detector based on detection result of the liquid leveldetector and the state detector and controlling a supply amount of to betreated solution stored in the distiller so that an actual liquid levelcoincides with the target liquid level, wherein the to be treatedsolution is provided to the cooler as a cooling medium, before thesolution is supplied to the distiller.
 2. The apparatus according toclaim 1, the apparatus further comprising a supply passage connected tothe cooler for passing the to be treated solution to the cooler as acooling medium, before the solution is supplied to the distiller.
 3. Theapparatus according to claim 1, wherein a concentrated solutioncontaining a residue is stored in the distiller; the apparatus furthercomprising a metering discharge mechanism for adjusting a dischargeamount of the concentrated solution so that the residue is dischargedfrom the distiller at a predetermined ratio relative to a distillate. 4.The apparatus according to claim 1, wherein the to be treated solutioncontains sulfuric acid, and the distiller includes a concentratingcolumn to distill the to be treated solution to form a high concentratedsulfuric acid solution.
 5. The apparatus according to claim 1, furthercomprising a vacuum pump for reducing an internal pressure of thedistiller.
 6. The apparatus according to claim 1, wherein a concentratedsulfuric acid solution, containing sulfur dioxide, is discharged fromthe distiller; the apparatus further comprising a reactor for reactingthe sulfur dioxide with hydrogen peroxide to form sulfuric acid.
 7. Theapparatus according to claim 1, wherein concentrated sulfuric acidsolution and a residue, at least one of which contains sulfur dioxide,are formed in the distiller during distillation of the to be treatedsolution; the apparatus further comprising a reactor for reacting thesulfur dioxide, contained in the at least one of the concentratedsulfuric acid solution and the residue, with hydrogen peroxide to formsulfuric acid.
 8. The apparatus according to claim 1, wherein the to betreated solution contains nitric acid, and the distiller includes apurifying column to distill the to be treated solution to form a highconcentrated nitric acid solution.
 9. A distilling apparatus fordistilling a hydrogen peroxide-containing solution to be treated,comprising: a remover for removing a substantial part of the hydrogenperoxide from the to be treated solution; a distiller, connected to theremover, for storing therein the to be treated solution from which thehydrogen peroxide has been removed and distilling the solution to form adistillate, wherein the distiller includes a cooler for cooling adistillation gas formed by heating the to be treated solution in thedistiller, and wherein the distiller includes a plurality of partitionsretained in the distiller at equal intervals, each of the plurality ofpartitions having a plurality of pores; and a control unit forcontrolling a supply amount of to be treated solution stored in thedistiller based on a liquid level of the to be treated solution in thedistiller, wherein the to be treated solution is provided to the cooleras a cooling medium, before the solution is supplied to the distiller.10. The apparatus according to claim 9, further comprising a liquidlevel detector, which is inserted to the distiller, for detecting theliquid level of the to be treated solution stored in the distiller. 11.The apparatus according to claim 9, wherein a concentrated solutioncontaining a residue is stored in the distiller; the apparatus furthercomprising a metering discharge mechanism for adjusting a dischargeamount of the concentrated solution so that the residue is dischargedfrom the distiller at a predetermined ration relative to a distillate.12. The apparatus according to claim 9, wherein the distiller has astorage section for storing and heating to be treated solution, and theremover has a piping penetrating the storage section through which theto be treated solution containing hydrogen peroxide flows such the to betreated solution containing hydrogen peroxide is heated at the storagesection.
 13. The apparatus according to claim 9, wherein the distillerincludes a first distiller and a second distiller; the apparatus furthercomprising a connecting pipe for connecting the first distiller to thesecond distiller; and a heater provided on the connecting pipe forheating a concentrated solution flowing therethrough.
 14. The apparatusaccording to claim 9, wherein the remover includes a catalyst forremoving hydrogen peroxide from the to be treated solution.
 15. Theapparatus according to claim 9, wherein the to be treated solutioncontains sulfuric acid, and the distiller includes a purifying column todistill the to be treated solution to form a high concentrated sulfuricacid solution.
 16. The apparatus according to claim 9, wherein the to betreated solution contains nitric acid, and the distiller includes apurifying column to distill the to be treated solution to form a highconcentrated nitric acid solution.
 17. A distilling apparatuscomprising: a distiller which stores therein a solution and distills thesolution to form a distillate, wherein a gas containing nitric acid mistis generated when performing the distillation; a liquid level detectordisposed in the distiller which detects the liquid level of thesolution; a state detector for detecting a physical state of thesolution contained in the distiller, wherein state detector includes atemperature sensor, a concentration sensor, and a specific gravitysensor; a control unit for controlling a supply amount of solutionstored in the distiller based on detection results of the liquid leveldetector and the state detector; and an exhaust chamber for absorbingthe nitric acid mist contained in the gas.
 18. A distilling apparatuscomprising: a concentrating column for heating a waste liquid togenerate a vapor and condensing the vapor to generate a concentratedliquid, wherein the concentrating column filters the condensed vapor toseparate out impurities and removes water from waste liquid in theconcentrating column; a liquid level detector disposed in theconcentrating column which detects the liquid level of the wastedliquid; a state detector for detecting a physical state of the wastedliquid in the concentrating column, wherein state detector includes atemperature sensor, a concentration sensor, and a specific gravitysensor; a control unit for controlling a supply amount of wasted liquidin the concentrating column based on detection results of the liquidlevel detector and the state detector; and a purifying column coupled tothe concentrating column for heating the concentrated liquid, whereinthe purifying column removes residue from the heated liquid in thepurifying column and condenses the heated liquid to produce about a 90%purified waste liquid.